Cryptology ePrint Archive: Report 2009/080

CoSP: A General Framework For Computational Soundness Proofs

Michael Backes and Dennis Hofheinz and Dominique Unruh

Abstract: We describe CoSP, a general framework for conducting computational
soundness proofs of symbolic models and for embedding these proofs
into formal calculi. CoSP considers arbitrary equational theories and
computational implementations, and it abstracts away many details that
are not crucial for proving computational soundness, such as message
scheduling, corruption models, and even the internal structure of a
protocol. CoSP enables soundness results, in the sense of preservation
of trace properties, to be proven in a conceptually modular and
generic way: proving x cryptographic primitives sound for y calculi
only requires x+y proofs (instead of x*y proofs without this
framework), and the process of embedding calculi is conceptually
decoupled from computational soundness proofs of cryptographic
primitives. We exemplify the usefulness of CoSP by proving the first
computational soundness result for the full-fledged applied
pi-calculus under active attacks. Concretely, we embed the applied
pi-calculus into CoSP and give a sound implementation of public-key
encryption and digital signatures.

Note: Erata will be published at: http://crypto.m2ci.org/unruh/publications/erata/cosp-erata.pdf

Version history:

2009-02-18: Initial revision

2009-06-03: Support for signatures has been added. The proofs have been made more detailed and the introduction extended.

2009-10-26: In definition of CoSP protocols: Merged constructor and destructor nodes into one type, computation nodes. In Section 3: Added nonces to the message type for the encryption theory (were missing). Some corrections in the proofs of Section 3.